Lifesteal is, on its own, not a counter to Thornmail. It never was.
Lifesteal = Percentage of damage x ratio of (Thorn user's Armor + 100) to (Thorn attacker's MR + 100)
With current Thornmail in the above formula, a tank with 90 base Armor would need, against an AD with a balanced build (flat MR runes,
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upgrade/
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- 30% Lifesteal and 87 MR), 183 additional bonus Armor on top of Thornmail to negate all the ADCs Lifesteal - but merely for the scaling damage part of Thornmail to deny the entirety of the Lifesteal. This calculation does not account for the flat damage.
However, the ADC needs all three last items to require the tank to have even that much Armor. Without
, the tank would only need 83 Armor on top of Thornmail. Without
, he only needs 12.7 of Thorn's 100 Armor. And without
, the tank doesn't even need Thorn's own Armor!
So, I have a problem with people saying that you just need Lifesteal to outheal Thornmail - because, if you look at the numbers, it is simply not true. ADCs need three big items to counteract Thornmail - and using 7500+ gold to counteract 2200 gold is ok in my book.
The purpose of Thornmail is to deny its user as a valid Lifesteal Battery target. By adding Grievous Wounds to it, you would change that purpose from being a selfish effect (deny Lifesteal on yourself) to a teamwide effect (reduce Lifesteal on everyone).